Add testing of mipmaps to backend allocation test


Bug: skia:9091
Change-Id: I1d928d3180c47d22a41c69e169da00b598318d47
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214301
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed